1. Field of the Invention
The present invention relates to fast crystallizing semicrystalline polyester compositions useful in molding applications.
2. Description of the Prior Art
Polyethylene terephthalate (PET) is a semicrystalline, linear saturated thermoplastic polyester that is known for its outstanding mechanical and thermal properties. However, unmodified PET has a slow crystallization rate, especially at temperatures below 130.degree. C.
Polycyclohexylenedimethylene terephthalate (PCT) also is a semicrystalline, saturated thermoplastic polyester. PCT is known for its high melting point as well as for its outstanding mechanical attributes comparable to those of PET. Similar to PET, unmodified PCT compositions also exhibit slow crystallization kinetics.
For molding applications of crystalline or semi-crystalline thermoplastic polymers, from practical and economical points of view, it is desirable to utilize polymers that can be molded in water heated molds which, in general, can only provide a molding temperature range of from about 76.degree. C. (170.degree. F.) to about 115.degree. C. (240.degree. F.). Therefore, it is desirable for these polymers to start crystallize at as high a temperature as possible upon the cooling of the polymer melt which was fed into the mold and continue to crystallize during the cooling to as low a temperature as possible. As is known in the art, such crystallization characteristic may be represented by T.sub.cc and T.sub.ch, which are measured using a Differential Scanning Calorimeter (DSC). T.sub.cc is a measurement to determine at what temperature crystals first appear upon cooling from the melt. T.sub.ch is a measurement which indicates the temperature at which crystallization is no longer occurring upon cooling, and is determined by measuring the temperature at which crystals first appear upon heating an amorphous piece of a polymer. Therefore, it is desirable to provide PET compositions that have as high a T.sub.cc as possible and as low a T.sub.ch as possible, allowing crystal formation and growth over the widest possible temperature range.
In order to improve the crystallization characteristics of PET, a variety of additives have been utilized in the art. One important class of the additives is nucleating agents, which has the effect of elevating the T.sub.cc of polyalkylene terephthalates. A variety of inorganic and organic nucleating agents are disclosed, for example, in U.S. Pat. Nos. 4,357,268 to Vanderkooi et al., 4,327,007 to Vanderkooi et al., 4,322,335 to Nield, 3,619,266 to Kelkheim et al. and 4,551,485 to Ragan et al.
Another important class of the additives includes plasticizers. The plasticizers are known to have the effect of lowering the T.sub.ch of polyalkylene terephthalates. A variety of plasticizers have been disclosed in the prior art references. These include, for example, those materials disclosed in U.S. Pat. Nos. 4,327,007 to Vanderkooi et al., 4,731,404 to Haylock et al., 4,385,144 to Jones et al., 4,486,561 to Chung et al., 4,539,352 to Chung et al., 4,548,978 to Garrison, Jr., 4,223,113 to Bier et al., 4,435,546 to Bier et al., 4,486,564 to Deyrup and British Patent 1,224,684.
Although the prior art plasticizers promote desirable low-temperature crystallization characteristics in PET and PCT compositions, they may render the compositions not suitable for certain applications in that many of the conventional plasticizers do not chemically bond or have sufficient affinity to the polymer chains of the polyesters and, thus, do not form stable polyester compositions. Consequently, the conventional plasticizers tend to separate from and migrate out of the polyester compositions during post-compounding processes, such as molding processes, in the form of low molecular weight compounds that are volatile. Such unbound plasticizers which migrate to the surface of molded articles may degrade surface appearance and interfere with paintability and colorability. Furthermore, such unbound plasticizers may also volatilize to accumulate at various spots in the manufacturing equipments and facilities, leading to potential environmental and safety difficulties. Therefore, there remains a need to provide PET and PCT molding compositions that crystallize at a commercially practical range of molding temperatures and yet do not utilize volatile additives.